如何在Java中自定义方法统计字符串中指定字符的出现次数

需求 #

实现一个静态方法count，统计字符串str中属于字符串chars的字符的出现次数，需通过以下JUnit测试用例：

JUnit测试用例 #

@Test
public void test_count_bothEmptyString() {
    int expected = 0;
    int actual = CCStringsIfAndWhile.count("", "");
    
    assertEquals("Testing count - both empty string", expected, actual);
}

@Test
public void test_count_firstEmptyString() {
    int expected = 0;
    int actual = CCStringsIfAndWhile.count("", "abcdefghijklmnopqrstuvwxyz");
    
    assertEquals("Testing count - first is empty string", expected, actual);
}

@Test
public void test_count_secondEmptyString() {
    int expected = 0;
    int actual = CCStringsIfAndWhile.count("This is a test", "");
    
    assertEquals("Testing count - second is empty string", expected, actual);
}

@Test
public void test_count_one() {
    int expected = 1;
    int actual = CCStringsIfAndWhile.count("This is a test", "abc");
    
    assertEquals("Testing count - 'This is a test', 'abc'", expected, actual);
}

@Test
public void test_count_many() {
    int expected = 6;
    int actual = CCStringsIfAndWhile.count("This is a test", "sapqi");
    
    assertEquals("Testing count - 'This is a test', 'sapqi'", expected, actual);
}

@Test
public void test_count_upperAndLowerCase() {
    int expected = 7;
    int actual = CCStringsIfAndWhile.count("This is another test", "stpq");
    
    assertEquals("Testing count - 'This is another test', 'stpq'", expected, actual);
}

现有代码问题 #

你当前的代码存在几个关键错误：

1. 循环内部的return charCount++;会直接终止方法，导致无法遍历完整个字符串
2. else分支将charCount重置为0，会清空之前的统计结果，完全不符合需求
3. 缺少判断字符是否属于chars的核心条件

public static int count(String str, String chars) 
{
    int count = 0;
    int charCount = 0;
    while(count<str.length()) 
    {
        if() 
        {
            charCount++;
            return charCount++;
        }
        else {
            charCount=0;
        }
        count++;
    }
    return charCount;
}

修正后的代码 #

public static int count(String str, String chars) {
    int charCount = 0;
    
    // 边界处理：任意一个字符串为空，直接返回0
    if (str.isEmpty() || chars.isEmpty()) {
        return 0;
    }
    
    int index = 0;
    while (index < str.length()) {
        char currentChar = str.charAt(index);
        // 判断当前字符是否存在于chars中
        if (chars.indexOf(currentChar) != -1) {
            charCount++;
        }
        index++;
    }
    
    return charCount;
}

代码说明 #

• 边界处理：提前判断str或chars为空的情况，直接返回0，满足前三个测试用例的要求
• 字符存在性判断：使用chars.indexOf(currentChar) != -1检查当前字符是否在chars中（indexOf返回-1表示字符不存在）
• 统计逻辑：遍历str的每个字符，符合条件时累加计数，遍历完成后返回总次数

内容的提问来源于stack exchange，提问作者Hisham Souraya